diff --git a/libc/upstream-netbsd/README.txt b/libc/upstream-netbsd/README.txt
new file mode 100644
index 0000000..86af6eb
--- /dev/null
+++ b/libc/upstream-netbsd/README.txt
@@ -0,0 +1,9 @@
+This directory contains upstream NetBSD source. You should not edit these
+files directly. Make fixes upstream and then pull down the new version of
+the file.
+
+Note that code in the other 'netbsd' directory contains Android modifications.
+We should work towards getting as many of those changes as possible upstream
+and then losing those files in favor of pure upstream copies here instead.
+
+TODO: write a script to make this process automated.